全新java初学者实践教程10(java SE5.0版)
+3B^e%`NPm jdk5的集合类
d4@\5< E[N5vG< Hd)z[6u8eT c5~d^ 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
NPjh2 AJm 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
#$trC)? ~q [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
a&yIH;- ZZ 1s}TG [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
n^OWz4 DoV<p?U [font="Times](1) Set
HD"Pz}k4 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
mQ#E{{:H+ >y<yFO{ ([font="Times]2)[font="Times]List
K}^Jf; 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
Wl3jbupu _ ISo{>@a- 5X^bvW26 [font="Times] ([font="Times]3)[font="Times]Map
BzFD_A>j;_ [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
a|B^% XRU^7@Ylks 9d ZE#l!Q [font="Times] ([font="Times]4)[font="Times]Queue
slSQ \;CDA [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
2I!STP{ !l [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
PVBf' y?BzZ16\bL "X/cG9Lw 实践:[font="Times] Set举例
zPwU'TbF [font="Times] import java.util.*;
['F, [font="Times]public class SetExample {
G/tah@N[7 [font="Times] public static void main(String[] args) {
rSTc4m1R [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
,BuN]9# [font="Times] set.add("one");
-!]Ie4" [font="Times] set.add("second");
QW~-+BD [font="Times] set.add("3rd");
9:tvkl [font="Times] set.add(new Integer(4));
n ,<`.^ [font="Times] set.add(new Float( 5.0F ));
8 jom)a [font="Times] set.add("second");
**I9Nw!IH [font="Times] set.add(new Integer(4));
b"Ep?=*5 [font="Times] System.out.println(set);
~r~~0|= [font="Times] }}
qK
,mG{ ~i)O^CKq [font="Times]List举例:
m#[tY>Q[b [font="Times] import java.util.*;
;1Kxqpz_i [font="Times]public class ListExample {
IT \Pj_ [font="Times] public static void main(String[] args) {
oYWcX9R [font="Times] List list = new ArrayList();
$#V^CmW. [font="Times] list.add("one");
k^A Yg!~ [font="Times] list.add("second");
cE
x$cZRMI [font="Times] list.add("3rd");
9T*%CI [font="Times] list.add(new Integer(4));
Rg*zUfu5%o [font="Times] list.add(new Float( 5.0F ));
?H9F"B$a [font="Times] list.add("second");
G-FTyIP>' [font="Times] list.add(new Integer(4));
7h?yAgDv~ [font="Times] System.out.println(list);
p{:r4!*L [font="Times] }}
Ln;jB&t g*9jPwdG $"Oy } [font="Times]Map举例
;]<{<czc [font="Times]import java.util.Map;
FrSeR9b [font="Times]import java.util.HashMap;
a$p2I+lX [font="Times]import java.util.Iterator;
=2%EIZ0oW [font="Times]import java.io.FileReader;
\!8`kC [font="Times]
.ON+ (
#n [font="Times]public class MapExample {
vfT<%Kl!' [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
G@Dw [font="Times] Map word_count_map = new HashMap();
0`X%& [font="Times] FileReader reader = new FileReader(args[0]);
1\d$2N" [font="Times] Iterator words = new WordStreamIterator(reader);
7#Qa/[? D [font="Times]
-C$Z%I7 0 [font="Times] while ( words.hasNext() ) {
/*GRE#7S [font="Times] String word = (String) words.next();
)Qve[O [font="Times] String word_lowercase = word.toLowerCase();
<@CBc:j0 [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
9E{Bn# [font="Times]if ( frequency == null ) {
#&\hgsw/T [font="Times] frequency = new Integer(1);
tK&.0)*= [font="Times] } else {
)2X ng_, [font="Times] int value = frequency.intValue();
S3"js4a [font="Times] frequency = new Integer(value + 1);}
M%7H-^{ [font="Times] word_count_map.put(word_lowercase, frequency);
!M~p __ [font="Times] }
YBgHX [q [font="Times] System.out.println(word_count_map);
s(7'*`G"h [font="Times] }}
Fz+0 h" ;K?fAspSH U5mec167
[font="Times]Queue举例:
?V_Qa0k [font="Times]import java.io.IOException;
"m]"%MU78 [font="Times]import java.io.PrintStream;
WG
9f>kE [font="Times]import java.util.LinkedList;
to Ei4u)m [font="Times]import java.util.Queue;
hoASrj{s [font="Times]
_t:cDXj [font="Times]public class QueueTester {
o"^}2^)_SR [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
qQR>z [font="Times] public QueueTester() {q = new LinkedList();}
;%
*e}w0 [font="Times]public void testFIFO(PrintStream out) throws IOException {
RM53B [font="Times] q.add("First");
z;x`dOP [font="Times] q.add("Second");
amf=uysr [font="Times] q.add("Third");
Co8b0-Z [font="Times] Object o;
=$5[uI2 [font="Times] while ((o = q.poll()) != null) {
*?oQ6g(Nz [font="Times] out.println(o);}}
v8Nc quv [font="Times] public static void main(String[] args) {
6wzF6]@O [font="Times] QueueTester tester = new QueueTester();
zTY|Z@: [font="Times] try { tester.testFIFO(System.out);
4 'rWy~`
V [font="Times] } catch (IOException e) {
fiTMS: [font="Times] e.printStackTrace(); } }}
fmie,[ jG{}b6 S>7Zq5* 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
my")/e 总结:
$JmL)r 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
@6GM)N\{[ 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。